The present invention relates to carrier compositions, and more specifically, coated carriers that are substantially free or free of conductive components like conductive carbon blacks. Examples of utility for the aforementioned carriers include their incorporation into copying and printing machines, such as xerographic machines, multifunctional devices, color systems, and the like, and wherein the coated carriers can be economically generated, for example, the blending of a carrier core and micropowder coating.
The electrostatographic process, and particularly the xerographic process, is known. This process involves, for example, the formation of an electrostatic latent image on a photoreceptor, followed by development of the image with a developer, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive developer particles are selected depending on the development systems used. Moreover, of interest with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that may enable continued formation of developed images of high quality and excellent resolution. In two component developer compositions, carrier particles are used in charging the toner particles.
Carrier particles in part are comprised, for example, of a roughly spherical core, often referred to as the xe2x80x9ccarrier corexe2x80x9d, which may be generated from a variety of materials, or purchased. The core is typically coated with a resin, such as a polymer or copolymer, and which resin may contain a conductive component, such as certain carbon blacks, to, for example, provide carrier particles with more desirable and consistent triboelectric properties. Including conductive components in the carrier coating may be disadvantageous in some instances, for example, it can be difficult and costly to blend the core and conductive component, and also the conductive component may not fully serve its purposes. For example, processes of incorporating conductive material into carrier coating include the use of electrostatic attraction, mechanical impaction, in situ polymerization, dry blending, thermal fusion and others, and which processes often result in only minimal amounts of conductive material being incorporated into the coating or there is generated conductive carrier coatings too large for effective and efficient use especially with smaller sized carriers. Additionally, dry blending processes and other mixing to incorporate the carbon black or other conductive material into the polymer coating can be selected, however, to avoid, or minimize transfer of the carbon black from the polymer coating the amount of carbon black that may be blended may be limited, for example, to 20 percent by weight or less, which limits the conductivity achievable by the resultant conductive polymer. Also, the carbon black from the carrier coating polymer can contaminate the toner resulting in charges in both charging performance and color of the toner, such as for example, a light colored toner, such as yellow.
In addition to the problems associated with loading conductive materials into coating resins, recent efforts to advance carrier particle science have focused on the attainment of conductive coatings for carrier particles to improve development quality, and provide particles that can be recycled and that do not adversely affect the imaging member in any substantial manner. Many coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes causing failure upon impact or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles, thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate.
A further problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. High relative humidity hinders image density in the xerographic process, may cause background deposits, leads to developer instability, and may result in an overall degeneration of print quality. In the science of xerography, the term xe2x80x9cA Zonexe2x80x9d refers, for example, to hot and humid conditions, the term xe2x80x9cC Zonexe2x80x9d refers, for example, to cold and dry conditions. Triboelectric charges are usually lower in the xe2x80x9cA Zonexe2x80x9d than in the xe2x80x9cC Zone.xe2x80x9d It is desirable to have the measured triboelectric charges (tc) for a particular carrier in the A Zone and the C Zone, when entered into a ratio of A zonetc/C zonetc to be close to about 1 to obtain development in high humidity.
A carrier coating commonly used is a polymethyl methacrylate (#MP-116 PMMA) available from Soken Chemical of Japan. This powder typically has a diameter of about 0.4 to 0.5 micrometer and it can be generated from polymethyl methacrylate. Usually high amounts of PMMA are selected for the coating of a 30 to 50 micrometer carrier core and to achieve surface area coverage on the carrier of about 85 percent to 95 percent. The use of such high amounts of carrier coating often results in lower carrier yields because, for example, of the formation of fused aggregates. Fused aggregates usually need to be broken up or removed by screening. Crushing or breaking up of the aggregates may result in weak or xe2x80x9cchipped offxe2x80x9d areas on the carrier surface potentially causing poor coating quality. Screen separation may result in a lower yield as aggregates are removed from the final product.
The above disadvantages and problems can be avoided or minimized with the coated carriers of the present invention in embodiments thereof. More specifically, there is a need to enable the effective, economical incorporation of carrier polymers, especially at certain coating weights, onto carrier cores to permit conductive carriers without the use of an additional conductive components while providing for and maintaining desirable xerographic qualities such as high coating efficiency, proper performance in both the A Zone and the C Zone, and stable charging characteristics.
Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may consist of various cores, including steel, with a coating thereover of fluoropolymers and terpolymers of styrene, methacrylate, and silane compounds.
There is illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there is disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight based on the weight of the coated carrier particles of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320xc2x0 F. to about 450xc2x0 F. for a period of about 20 minutes to about 60 minutes, enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent, the disclosure of which is incorporated herein by reference in its entirety, are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics. Further, in regard to U.S. Pat. No. 4,233,387, it is believed that only certain triboelectric charging values can be generated, when certain conductivity values or characteristics are contemplated.
U.S. Pat. No. 4,937,166, the disclosure of which is totally incorporated herein by reference, describes a carrier composition comprised of a core with a coating thereover comprised of a mixture of first and second polymers that are not in close proximity thereto in the triboelectric series. The core is described to be iron, ferrites, steel or nickel. The first and second polymers are selected from the group consisting of polystyrene and tetrafluoroethylene; polyethylene and tetrafluoroethylene; polyethylene and polyvinyl chloride; polyvinyl acetate and tetrafluoroethylene; polyvinyl acetate and polyvinyl chloride; polyvinyl acetate and polystyrene; and polyvinyl acetate and polymethyl methacrylate. The particles are described to, for example, possess a triboelectric charging value of from about xe2x88x925 to about xe2x88x9290 microcoulombs per gram.
U.S. Pat. No. 4,935,326, the disclosure of which is totally incorporated herein by reference, discloses a carrier and developer composition, and a process for the preparation of carrier particles with substantially stable conductivity parameters which comprises (1) providing carrier cores and a polymer mixture; (2) dry mixing the cores and the polymer mixture; (3) heating the carrier core particles and polymer mixture, whereby the polymer mixture melts and fuses to the carrier core particles; and (4) thereafter cooling the resulting coated carrier particles. These particulate carriers for electrophotographic toners are described to be comprised of core particles with a coating thereover comprised of a fused film of a mixture of first and second polymers which are not in close proximity in the triboelectric series, the mixture being selected from the group consisting of polyvinylidene fluoride and polyethylene; polymethyl methacrylate and copolyethylene vinyl acetate; copolyvinylidene fluoride tetrafluoroethylene and polyethylenes; copolyvinylidene fluoride tetrafluoroethylene and copolyethylene vinyl acetate; and polymethyl methacrylate and polyvinylidene fluoride.
U.S. Pat. No. 5,567,562, the disclosure of which is totally incorporated herein by reference, describes a process for the preparation of conductive carrier particles which comprises mixing a carrier core with a first polymer pair and a second polymer pair, heating the mixture, and cooling the mixture, wherein the first and second polymer pair each contain an insulating polymer and a conductive polymer, and wherein the carrier conductivity thereof is from about 10xe2x88x926 to about 10xe2x88x9214 (ohm-cm)xe2x88x921. The first polymer pair is preferably comprised of an insulating polymethyl methacrylate and a conductive polymethyl methacrylate, and the second polymer pair is preferably comprised of an insulating polyvinylidene fluoride and a conductive polyvinylidene fluoride.
There are illustrated in U.S. Pat. No. 6,042,981, the disclosure of which is totally incorporated herein by reference, carriers including a polymer coating wherein the polymer coating may contain a conductive component, such as carbon black, and which conductive component is preferably dispersed in the polymer coating. The conductive component is incorporated into the polymer coating of the carrier core by combining the carrier core, polymer coating, and the conductive component in a mixing process such as cascade roll mixing, tumbling, milling, shaking, electrostatic powder cloud spraying, fluidized bed, electrostatic disc processing or by an electrostatic curtain. After the mixing process, heating is initiated to coat the carrier core with the polymer coating and conductive component.
There are illustrated in U.S. Pat. No. 5,928,830, the disclosure of which is totally incorporated herein by reference, processes for the preparation of black and color toner compositions. This toner is prepared from latex comprised of a polymer core and a polymer shell thereover. The latex is fused or coalesced together forming toner particles comprised of an aggregation of the latex. Conductive or pigment components may incorporated into the latex cores/shell prior to coalescence by blending a dispersion of the conductive or pigment component in a cationic surfactant with the latex core/shell in an ionic or nonionic surfactant. Specific examples of conductive components incorporated in U.S. Pat. No. 5,928,830 include conductive carbon black SC Ultra available from Conductex Inc. and antimony-doped tin oxide Zelec ECP3005-XC manufactured by E.I. DuPont.
It is a feature of the present invention in embodiments to provide conductive carriers capable of achieving specific levels of conductivity and high surface area coverage.
It is another feature of the invention in embodiments to provide effective and economical processes for achieving conductive carriers while avoiding conductive components, like carbon black.
It is a further feature of the invention in embodiments to provide coated xerographic carriers that possess, for example, excellent xerographic properties in both the A zone and the C zone for use in toner and developer compositions.
Illustrated herein are carrier micropowders. The micropowder contains, as the primary resin, polymer particles recovered from a latex emulsion polymerization or copolymerization with a surfactant. Conductive materials, and optionally charge enhancing additives, can also be incorporated with the polymer powder to form the micropowder. The micropowder may then be incorporated onto the surface of carrier cores providing in embodiments coated carriers with conductive characteristics, and wherein the carriers are coated with a number of specific polymers, such as SLS (sodium dodecyl sulfate) the surfactant used in the emulsion-polymerization process and TFEMA-MMA a copolymer of trifluoroethyl methacrylate and methyl methacrylate.